Pokerus
Pokerus is a beneficial virus that one of your Pokemon can catch. You catch it by battling Pokemon in wild. If the wild Pokemon has it, then the Pokemon that battles will catch it. Pokerus is good thing, though, because it will double Effort Values you get from battling. So a regular Pokemon battling a Bidoof will get 1 HP Effort Value, While a Pokerus infected Pokemon will 1 2 HP Effort Values. The person at the Pokemon Centre will notice when you have Pokerus.

You can spread Pokerus to all the Pokemon in your team. You can have your entire team gain double Effort Values if you want. Once a pokemon heals and no longer has it, they will still gain double Effort Values, the just won't be able to spread it. A Purple box with PKRS will be the stat page of your Pokemon when you Pokemon has it. When it goes away it will be replaced with a Smiley Face.